During printing, ink is deposited on a substrate surface; the substrate being, for example, paper or plastic. It is then common to cover the printed surface of the substrate with a protective layer. The protective layer completes the fixing of the image printed on the substrate and substantially protects the printing against certain external threats such as, for example, projections, as well as light, heat and/or moisture. It is known to use an ink-jet. This protective layer on the printed substrate is generally deposited by an ink-jet printing. The varnish also makes it possible to customize the document by revealing areas with designs in varnish and other areas without varnish.
U.S. Pat. No. 7,279,506, filed May 5, 2004, Ser. No. 10/838,212, discloses an example of an overprint ink deposited by an ink-jet printer, where the overprint ink composition includes oligomers and monomers hardenable by radiation and at least one photoinitiator to initiate polymerization and thus the rigidification of the deposited overprint layer. This document proposes a diversified selection of a number of components to produce such an overprint ink. However, the choice of these components and their mixture is dictated in the proportions of oligomers and monomers which yield an overprint ink whose viscosity lies between 15 mPa·s at 40° C. and 8 mPa·s at 70° C. An overprint ink with such a viscosity requires heating the print heads to temperature ranges high enough to avoid agglomeration of the varnish. Unfortunately, this causes the print heads, which are relatively expensive parts of the printing devices, to deteriorate more rapidly than if they were operated at a lower temperature.
The present invention aims to overcome one or more disadvantages of the prior art by providing a protective overprint ink for use with ink-jets, wherein the ink reduces the rate of deterioration of ink-jet print heads and that functions in temperature ranges that do not require significant heating of the overprint ink (frequently referred to herein as a varnish) during its deposition on a printed substrate.